Experimental study on the influence of the thermal input on the reaction zone under flameless oxidation conditions

The reaction zone of a small-scale laboratory combustor operating under flameless oxidation conditions is examined with the aid of hydroxyl radical chemiluminescence (OH*) imaging and measurements of local mean gas temperatures and local mean major gas species (O2, CO2, CO, unburnt hydrocarbons and NOx) concentrations along the combustor axis, as a function of the fuel (methane) thermal input, which was varied between 7 and 13kW. As the fuel thermal input increases, the reaction zone, as typified by the OH* distribution, enlarges and, simultaneously, moves progressively closer to the combustor exit (exhaust) due to the increase in the central jet momentum, while maintaining constant the excess air level. The excess air values used in the present study were low enough to preserve the flameless combustion regime regardless of the fuel thermal input, with the combustor yielding very low NOx (